docs: explain TCP NAT presence and IP-only addresses

src/routing_layer/topo_node.h

@@ -144,6 +144,7 @@ struct TOPOMSG_NODE {
 #define TOPOMSG_NODE_HDR_SIZE sizeof(struct TOPOMSG_NODE)
 
 struct TOPO_SOCKMETA4 { struct TOPO_SOCKMETA4* next; uint8_t id, config_type; struct address_profile address_profile; };
+/* port=0 — известен только внешний TCP IP; адрес хранится, но не участвует в подключении. */
 struct TOPO_ADDR4     { struct TOPO_ADDR4* next; uint8_t addr[4]; uint16_t port; uint8_t type, socket_id, protocol; };
 struct TOPO_SOCKMETA6 { struct TOPO_SOCKMETA6* next; uint8_t id, config_type; struct address_profile address_profile; };
 struct TOPO_ADDR6     { struct TOPO_ADDR6* next; uint8_t addr[16]; uint16_t port; uint8_t type, socket_id, protocol; };